THE MANOMETERS. 33 



ing by the number of settings less one, we obtain a mean correction 

 for a double meniscus, which is the meniscus correction to be applied 

 in all measurements of pressure, since the nitrogen in the manometers 

 is always included between two mercury columns. 



The method which is explained above suffices for the simple form of 

 manometer seen in Figure 18, but some modifications are necessary 

 when a manometer of the form seen in Figure 19 is to be calibrated. 

 The peculiarity of the latter instrument is the large reservoir for gas 

 which lies between the two lines of reference and within the calibrated 

 area. The narrower portions — below, from some point under the lower 

 scratch to the bottom of the enlargement, and above, from the 

 top of the wide part to the end of the tube — are calibrated in the man- 

 ner already described. The meniscus correction also is derived from 

 the weight and length relations of short and long threads. So far the 

 procedure is without change. It remains, however, to ascertain the 

 capacity of the wider part as a whole, and eventually in terms of 

 the calibration unit. To do this, the wider part is slightly more than 

 filled with mercury, so that both the upper and lower meniscus are well 

 within calibrated portions of the narrow ends. From the weight of 

 this mercury — with proper correction for overlapping in the narrower 

 calibrated parts— the total capacity of the wider part of the tube is 

 calculated. Two verifications of the correctness of the previous work 

 are now undertaken. It will be noticed, on referring to Figure 19, 

 that the upper line of reference is not very far above the upper end 

 of the wider portion of the manometer. The first step in the verifica- 

 tion is to fill the space between the two scratches with mercury — the 

 upper meniscus may lie somewhat above the upper scratch. The 

 volume of this mercury should, of course, be equal to the sum of the 

 previously found capacities of all of the parts which were filled by it. 

 The final step in the verification is to apply the same test to the whole 

 tube by filling it with mercury from the lower scratch to the upper 

 limit of the calibration. 



THE MENISCUS. 



In narrow tubes, owing to the small volume of the gas which they 

 contain, the meniscus correction is of considerable importance, since 

 it may amount — especially at high pressures — to an appreciable frac- 

 tion of the volume of the gas. 



The significance of the meniscus correction, when translated into 

 pressure, increases with increasing concentration of the solutions with 

 a rapidity which might well astonish one who has not clearly in mind 

 the fact that, though in the first instance it is simply a space of fixed 

 volume, its importance depends, not only on the pressure upon the 

 gas which fills it, but also upon the volume of all the gas in the manom- 

 eter. The effect of this relation in practice is illustrated by means of 



JUJ' L I P 



